1. Field of the Invention
The present invention relates to load elevators for use in loading and unloading a number of packages or objects, and more specifically, to load elevators and methods of using them that can maintain the top of a changing load at a convenient predetermined height.
2. Description of Related Art
In the handling of a number of packages or objects, a common task is to manually transfer them between an elevated table, conveyor, etc. and a pallet resting on the floor. While the table or conveyor remains at a fixed height, the top of the load on the pallet, where the next box or object is to be loaded or removed, usually is at a different height which varies as the packages, etc. are piled on or removed from the pallet. This difference in height, and the changes in this difference during loading or unloading of the packages, can be fatiguing for the person doing the moving. Therefore, elevators have been developed for raising the pallet from the floor to a more convenient height and even for automatically adjusting the height of the pallet as the load increases or decreases so the preferred height of the top of the load is maintained.
For example, Cox's U.S. Pat. No. 4,764,075 shows a scissor elevator supported by helical metal springs that maintain the top of a load of boxes or the like at a preset height P above the floor (e.g. 0.975 m) as boxes are added or removed (Cox, FIG. 6, upper diagram). Once the desired preset height P, the maximum load height h and maximum load weight W, and the height S of the fully compressed springs are given, only the spring constant k (or the value of the spring capacity C=k.multidot.S) needs to be determined. Because the maximum load height h and maximum weight W will vary from application to application, such a device usually needs to be purchased with a number of sets of springs and a change in application requires the consulting of a spring table and inconvenient changing of springs. That is, the means for adjusting the elevator for changes in the maximum load weight or changes in the density of the loaded objects is crudely quantized into cumbersome changes of the elevator springs rather than being continuously variable. Also, it is not practical to make such adjustment, i.e. the change of springs, while the elevator is under load.
While theoretically the spring constant k (or spring capacity C) of Cox's device might be determined by the user experimentally for a given maximum load height h and maximum weight W, in practice it is impractical to switch springs with a load in place. Therefore, Cox provides a formula or table for the user to follow to estimate the needed spring capacity C from the maximum load height h and maximum weight W. The formula assumes that the load is of approximately uniform density from box to box, in which case the springs should ideally be linear springs. The formula also assumes that, if there is no load on the pallet, the selected spring should produce only a little preloaded force to support the empty elevator mechanism and pallet.
In contrast, French Patent No. 1,473,991 (Fogautolube S. A.) shows a scissor-type pneumatic jack powered by a double-chamber bellows inserted between a base plate 2 and an intermediate plate or cup carried by the pivot pin of the scissors. However, this device appears to have been designed as a simple jack for autos and therefore does not discuss how to automatically keep loads of different heights at a preset distance from the floor for loading and unloading. Because the shape of a bellows changes significantly when substantially inflated or compressed, such a device may be less linear than a spring. However, substantial linearity is not required for a lifting jack because one simply inflates the bellows until the desired height is reached.
Similarly, French Patent No. 1,282,755 (Dittberner) shows a scissor-type pneumatic jack powered by a single-chamber bellows inserted between a base plate and an intermediate plate or cup carried by the pivot pin of the scissors.
German Patent No. 3,801,491 (Hahn) shows a scissor-type pneumatic jack powered by a single-chamber bellows inserted between plates carried by the scissor legs. This device is a loader/receiver for shelf system, and therefore it features a way of tilting the upper platform.
U.S. Pat. No. 3,174,722 (Alm) also is a jack for lifting vehicles. It has a scissor-type pneumatic lifter powered by a bellows inserted between upper and lower plates which ride in tracks on the scissor legs. U.S. Pat. No. 4,921,074 (Ochs) shows a narrow scissor-type pneumatic lifter attachment that can be fitted on each track of existing vehicle lifting platforms (See FIG. 1). The bellows is between a base and a top platform, but to one side of the scissor support (FIG. 2) so that the device is not wider than a vehicle track.
U.S. Pat. No. 4,688,760 (Garman) has a scissor-type pneumatic lifter powered by a multi-chamber bellows inserted between an upper platform and the base. Since the upper platform has a "backrest assembly 60" it appears to be designed as a lifter chair or bed. Its novel features are directed an improved linkage for the scissor legs which stabilize the top platform.
Other jack patents simply show various geometries for scissor-type automobile-lifting pneumatic jacks. For example, U.S. Pat. No. 2,070,960 (Phillips) is a compact lift having a multi-chamber bellows surrounded by scissors on four sides. U.S. Pat. No. 2,610,824 (Grier) has a plurality of bellows or air bags, each bag having a metal frame with holes 34 to accommodate the pivot pin of a corresponding scissor or "lazy tong" stabilizer. U.S. Pat. No. 3,379,411 (Vanderjagt) uses an air hose folded on itself several times as the bellows, and a lifting arm (claim 3) or plate (claim 4) is connected to one end of a base 22. U.S. Pat. No. 2,001,744 (Patterson) shows front and side scissor stabilizers around a plurality of bellows.
While the bellows devices of the prior art have the advantage that their elasticity or springiness can be increased or decreased by respectively raising or lowering the air pressure in the bellows, they have the disadvantage of being generally less linear than the sets of helical springs of Cox over a wide range of potential loads. Moreover, when a bellows is inflated or deflated with high pressure air, it may suddenly elongate or contract. If such a bellows were used as an actuator in a load elevator, such sudden elongation or contraction could translate into sudden motion of the elevator that could injure the person it.
Therefore, it is an object of the present invention to provide a self-adjusting load elevator that uses a bellows as the height adjusting device yet overcomes these disadvantages. It is a further object of the invention to provide such a load elevator with a safety mechanism for preventing sudden or jerky motion that may be dangerous to the user. A further object is to provide improved methods of using such devices that make it easy for the user to adjust the load elevator for different maximum loads.